Pressure supply systems such as hydraulic or pneumatic systems are operated by a pressure pump at a rated system pressure. When such systems are used in clamping operations, for example to hold a workpiece in a machine tool, it is often necessary to adjust the system pressure to a given situation requiring a different rated system pressure which may be higher or lower than the initially adjusted rated system pressure. Changing the rated system pressure, however, introduces a relatively large source of error or fault, since at least two elements that influence each other must be adjusted.
Such pressure supply systems, especially hydraulic systems, require a pressure relief valve, which limits the actual pressure to a maximal system pressure and simultaneously determines the safety of the system. Conventionally, a pressure switch governs the pressure generating pump, or rather, a motor driving the pump. The pressure relief valve controls the maximal pressure in the system by bleeding excess pressure fluid to an overflow line when the pressure exceeds a rated pressure. If it becomes necessary to change the system pressure, then the pressure relief valve and the pressure switch must be adjusted relative to each other in their response characteristic. Even minor errors in the adjustment can result in damage to system components or to pressure-driven devices connected to the system, such as workpiece holding clamps. In addition, the pressure switch functions imprecisely in the lower pressure range and with a large hysteresis. Adaptation of the system pressure of such a system to a new rated system pressure thus requires that the operating personnel have a great deal of expertise and experience in making such adjustments. Consequently, adjusting the system pressure can be problematic in many cases.
Prior art teaches mechanically coupling the pressure relief valve and the pressure switch to each other by arranging the valve and the switch on a common shaft. The disadvantage of such an arrangement is that a mechanical adjustment across greater pressure ranges is often inaccurate since the spring forces of the elements, i.e. in the pressure relief valve and in the pressure switch, differ. Different pressures are effective on these two elements because of their different tasks. As a result, differential forces flow into the mechanics, thus making it quite difficult and time-consuming to achieve an exact adjustment.